Weight gain with aging and its potential resultant obesity are significant health concerns in older individuals. Evidence suggests that compensatory changes in energy expenditure (EE) occur in response to periods of altered energy balance, which nnay be pronounced in older adults. Inducing an energy deficit by exercise is an important strategy to treat obesity; however, it is not known whether there is an optimal exercise prescription that maximizes total daily EE, perhaps by minimizing the reduction in activity EE other than the energy expended during exercise program that may occur in response to exercise participation. Moreover, the physiological mechanisms regulating EE changes in response to altered energy balance are not known. The study presented in this ROO application will help to address these questions by determining whether dose of exercise programs influences changes in EE and potential energy-regulatory biological factors in older women. The specific aims of the proposed research are to determine whether differential changes occur in total daily EE and its components in response to exercise programs of two different doses. Women (60-75 years) will be randomized to 16-week aerobic exercise programs of 8 or 14 kcal/kg body weight per week. State-of-the-art techniques will be used to measure total daily EE (using doubly labeled water) and its components (resting metabolic rate, thermic effect of feeding via indirect calorimetry, and non-exercise activity EE using the Physical Activity Monitoring System) at baseline and post-intervention. We hypothesize that women will exhibit a smaller increase in total daily EE in response to the higher-dose, compared to the lower-dose exercise program, due to a greater decline in non-exercise activity EE. We will also determine whether differential changes occur in blood concentrations of potential energy-regulatory factors, such as leptin and free T3, and variables related to cardiometabolic diseases, including plasma lipids, insulin sensitivity, blood pressure, aricj aerobic fitness after the exercise programs of different doses.